As for high-property magnet with (BH)max exceeding 40 MGOe used in various high-performance electric motor or electric generator, it is extraordinarily necessary for the development of “low-B component magnet” by decreasing the usage of non-magnetic element B in order to obtain a highly magnetization magnet.
At present, the development of “low-B component magnet” has adopted various manners; however, no corresponding marketized product has been developed yet. The greatest disadvantage of “low-B component magnet” lies in the deterioration of the squareness (also known as Hk or SQ) of the demagnetizing curve. The reason is rather complicated, which is mainly owing to the partial lack of B in the grain boundary caused by the existence of R2Fe17 phase and the lack of B-rich phase (R1.1T4B4 phase).
Japanese published patent 2013-70062 discloses a low-B rare earth magnet, which comprises R (the R is at least one rare earth element comprising Y, Nd is an essential component), B, Al, Cu, Zr, Co, O, C and Fe as the principal component, the content of each element is: 25˜34 weight % of R, 0.87˜0.94 weight % of B, 0.03˜03 weight % of Al, 0.03˜0.11 weight % of Cu, 0.03˜0.25 weight % of Zr, less than 3 weight % of Co (does not contain 0 at %), 0.03˜0.1 weight % of O, 0.03˜0.15 weight % of C, and the balance being Fe. In the invention, by decreasing the content of B, the content of B-rich phase is decreased accordingly, thus increasing the volume ratio of the main phase and finally obtaining a magnet with a high Br. Normally, when the content of B is decreased, R2T17 phase with soft magnetic property (generally R2T17 phase) would be formed, the coercivity (Hcj) of the magnet would be extremely easily decreased consequently. But in the invention by adding minor amounts of Cu, the precipitation of R2T17 phase is suppressed, and further forming R2T14C phase (generally R2Fe14C phase) which improves Hcj and Br.
However, the above stated invention still fails to solve the inherent problem of low squareness (Hk/Hcj, also known as SQ) of the low-B magnet; it can be seen from the embodiments of the invention, Hk/Hcj of only a few embodiments of the invention exceeds 95%, Hk/Hcj of most of the embodiments is around 90%, further none of the embodiments reach over 98%, only in terms of Hk/Hcj, it is usually difficult to satisfy the requirements of the customer.
To explain it in detail, if the squareness (SQ) deteriorates, the heat-resistance of the magnet would also deteriorate consequently even when the coercivity of the magnet is rather high.
Thermal demagnetization of magnet happens when the electric motor rotates in high load, consequently the electric motor could not rotate gradually, further stop working. Therefore, there are a lot of reports related to develop a high coercivity magnet with “low-B component magnet”, however, the squareness of all of the above stated magnet is not satisfying, which may not solve the problem of thermal demagnetization in the actual heat-resistance experiment of the electric motor.
In conclusion, no precedent of a “low-B component magnet” becomes the product actually accepted by the market.
On the other hand, the maximum magnetic energy product of Sm—Co serial magnet is approximately below 39 MGOe, therefore the NdFeB serial sintered magnet with the maximum magnetic energy product of 35˜40 MGOe selected as the magnets for the electric motor or electric generator would occupy a large market share. Especially on the basis of reducing the CO2 emission and the crisis of oil depletion, the pursuit of high efficiency and power-saving characteristics of the electric motor or electric generator is more and more severe, and the requirement for maximum magnetic energy product of the magnet for the electric motor and electric generator is higher and higher.